Method of monitoring deposition of a noble metal in a nuclear reactor and deposition monitor therefor

ABSTRACT

In a method of monitoring deposition of a noble metal in an intergranular stress corrosion crack (IGSCC) in a metal reactor shroud wall of a nuclear reactor, a metal sample may be placed at a location within near an inner surface of the metal reactor shroud wall. The sample may be submerged below a water line in the reactor and includes at least one thermal fatigue crack. The sample is maintained at the location for a given duration, and a given amount of the noble metal is added into the reactor water while the sample is maintained at the location. The sample is then removed. In an example, a surface crevice deposition monitor for a reactor includes a flow conditioner arranged between a top guide clamp and an anchor clamp, and at least one sample holder connected between the top guide clamp and flow conditioner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Example embodiment(s) of the present invention are related in general toa method for monitoring disposition of a noble metal in an intergranularstress corrosion crack (IGSCC) in a metal reactor shroud wall of anuclear reactor, and to a surface crevice deposition monitor for thereactor.

2. Description of the Related Art

Typically, interior surfaces of a metal reactor shroud wall of a nuclearreactor may be susceptible to the formation and/or propagation of one ormore IGSCCs that form during operation of the nuclear reactor.

BRIEF DESCRIPTION OF THE INVENTION

An example embodiment of the present invention is directed to a methodof monitoring deposition of a noble metal in an intergranular stresscorrosion crack (IGSCC) in a metal reactor shroud wall of a nuclearreactor. In the method, a metal sample may be placed at a locationwithin near an inner surface of the metal reactor shroud wall. Thesample may be submerged below a water line in the reactor and includesat least one thermal fatigue crack. The sample is maintained at thelocation for a given duration, and a given amount of the noble metal isadded into the reactor water while the sample is maintained at thelocation. The sample is then removed.

Another example embodiment of the present invention is directed to asurface crevice deposition monitor for a reactor. The monitor includes aflow conditioner arranged between a top guide clamp and an anchor clamp,and at least one sample holder connected between the top guide clamp andflow conditioner.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present invention will be more clearlyunderstood from the detailed description taken in conjunction with theaccompanying drawings/figures. The drawings/figures provided herein arefor illustrative purposes only. They illustrate various non-limitingaspects of various embodiment(s) of the invention. Other variations maybe possible. Also, as the figures/drawings are provided for illustrativepurposes, they may not be drawn to scale. Further, variousdrawings/figures may show optional equipment which is by definition notrequired for practicing the present invention.

FIG. 1 is a cross-sectional view of an interior wall/surface of a metalreactor shroud in a reactor, illustrating a sample where moleculesmimicking dissolved oxygen molecules and a noble metal penetrate intothe IGSCC of the metal reactor shroud, with the sample having at leastone or more TFCs formed therein.

FIG. 2 is an example sample holder assembly containing three samples, inaccordance with the example embodiments of the present invention.

FIG. 3 is a cut-out view of a portion of a metal reactor shroudincluding a sample holder.

FIG. 4 is a cut-away view showing only the sample holder without shroud.

FIG. 5 a is an enlarged view of a portion of the sample holder showinghow a sample may be held within a cavity of the sample holder.

FIG. 5 b shows a sub-holder portion of the sample holder in FIG. 5 a.

FIGS. 6 a-6 c illustrates the sample removed from the metal reactorshroud, where one or more cracks are split open so that the samplebreaks into two portions exposing an interior surface thereof.

FIG. 7 illustrates an example process of using a scanning electronmicroscope (SEM) to reveal the deposition of a noble metal on surfacesof the crack.

FIG. 8 is an SEM photograph (at 100,000×) of a test stainless steelsample.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

The following description of the invention is provided with respect tovarious non-limiting example embodiment(s) thereof. The followingdescription is not meant to unduly limit or restrict the presentinvention. Variations of the example embodiment(s) described herein arewithin the scope of the present invention.

In view of the foregoing, there is to be described in more detailhereafter a method of monitoring the extent to which an IGSCC is formedand/or propagated within the metal reactor shroud wall for safetyconcerns (e.g., if the cracks get too deep, the structural integrity ofthe metal reactor shroud may be decreased below a functional or safetylimit). Also, there is to be described a method of slowing down,stopping and/or partially or substantially repairing the IGSCC or itsformation and/or propagation within the metal reactor shroud wall.

In one example embodiment of the invention, a sample (e.g., a metallicsample or a sample made of other suitable material) is submerged beneatha surface level of the reactor coolant (e.g., water) contained withinthe metal reactor shroud. The sample is designed to include at least oneIGSCC. Such an IGSCC (in the sample) is referred to herein as a thermalfatigue crack (TFC) because the TFC is created so as to mimic an IGSCCthat may be formed and/or propagated within the metal reactor shroudwall. Thus, the TFC has been purposefully formed in the sample,according to one example embodiment of the invention.

The formed sample is then placed at a distance, for example, at about 6″or less from the interior surface of the metal reactor shroud wall. Thesample may be placed at other suitable distances (e.g., more than 6″away from the interior surface of the metal reactor shroud wall). In oneexample, a sample holder holding the sample may be positioned adjacentan inner surface of the shroud wall within about 4″ of the nearest fuelbundle (and/or within a range of about 4-10 inches from any fuel bundle)to hold the sample. In another example, the sample holder may be used tohold probes and/or other indicators or sampling devices, for example.

Over time, it is expected that the sample (e.g., a metal such as carbonsteel, stainless steel, low carbon stainless steel such as 304Lstainless steel, etc.) should substantially mimic thebehavior/performance of the interior surface of the metal reactor shroudwall as it relates to the formation and/or propagation of an IGSCC.Thus, for example, it is believed that if the sample shows excessiveformation and/or propagation of TFCs, that excess formation orpropagation should be a reflection of the extent of IGSCCs formed and/orpropagated in the metal reactor shroud wall itself.

In other words, the degree of TFC formation and/or propagation withinthe sample should to reflect the degree of IGSCC formation and/orpropagation within the metal reactor shroud wall. Thus, to monitor thestructural integrity of a metal reactor shroud over a period of time, itis possible to do so by closely inspecting the sample which had beenplaced and maintained in the metal reactor shroud at least partly duringthe reactor operation. Inspection is easier if performed after thesample is removed from the metal reactor shroud at anappropriate/convenient time interval. The removed sample is inspectedand analyzed for formation and/or propagation of TFCs.

According to another example embodiment of the invention, for example,typically when the nuclear reactor is shut down for maintenance purposes(known as an outage), at least one of the samples (if multiple samplesare used) may be removed and examined by a variety of methods including,but not limited to, acid stripping analysis or inspection under scanningelectron microscopy (SEM) or otherwise. Based upon examination of thesample crack (TFC) for the extent/degree to which a noble metal hasdeposited therein, the amount of the noble metal that may be injectedinto the reactor coolant may be adjusted (e.g., increased, decreased orotherwise altered) to counteract the formation and/or propagation ofIGSCCs in the metal reactor shroud wall itself. This is possible becausethe reactor coolant comes in direct contact with the interior surface ofthe metal reactor shroud.

It is believed that the noble metal diffuses into the cracks not onlywithin the sample TFCs but also within the cracks (IGSCCs) present(formed and/or propagated) within the metal reactor shroud wall itself.The noble metal ultimately may find its way into the IGSCCs and the TFCsand deposits itself therein, respectively. The noble metal may diffusedeeper into the IGSCC, well beyond where dissolved oxygen may penetrateinto the IGSCC itself. Thus, it is believed that the noble metal may beable to prevent, mitigate or slow down formation and/or propagation ofthe IGSCC, which is believed to be accelerated and/or worsened by thepresence of dissolved oxygen in the reactor coolant.

Typically, in examination by either acid stripping analysis, SEMinspection or otherwise, the amount of a noble metal that should bedeposited into the crack may be determined. Based upon the depth of thecrack (TFC), and/or the level of noble metal deposited within the crack(TFC), the level of noble metal that is injected into the reactorcoolant may be increased, decreased (or otherwise adjusted) as necessaryto mitigate, reduce, eliminate or otherwise reduce the formation and/orpropagation of an IGSCC within a metal reactor shroud wall. It isbelieved that the noble metal may penetrate much deeper into the IGSCCthan the dissolved oxygen.

In the figures, numbering is used consistently to refer to the samecomponent(s) or parts thereof. Note that an IGSCC 30 is modeled as to bedescribed in detail below by the thermal fatigue cracks (TFCs) referredto in FIGS. 4, 5 a, and 6 a to 6 c and labeled as cracks 30 a and 30 b.

FIG. 1 is a cross-sectional view of an interior wall/surface of a metalreactor shroud in a reactor, illustrating a sample where moleculesmimicking dissolved oxygen molecules and a noble metal penetrate intothe IGSCC of the metal reactor shroud, with the sample having at leastone or more TFCs formed therein.

Referring to FIG. 1, an internal surface 10 of a portion of a metalreactor shroud 20 is depicted with an illustration of an IGSCC 30. FIG.1 is a cross-sectional view of an interior wall of the metal reactorshroud showing flow direction 40 of reactor coolant (e.g., water)flowing past the IGSCC 30 within the thickness of the metal reactorshroud 20 wall. The flow direction of the coolant, while shown flowingfrom left to right, may be in an opposite (or other) direction as well.

The reactor coolant flowing within metal reactor shroud 20 containsdissolved oxygen molecules 33. These dissolved oxygen molecules 33typically could cause the IGSCC 30 to form and/or propagate deeper intothe cross-section of the metal reactor shroud wall. Arrows 31 and 32 mayillustrate movement of oxygen into the IGSCC 30. Over time, the crack 30typically continues to penetrate deeper into the metal reactor shroud20, for example, in the general direction of arrows 31 and 32. The IGSCC30 typically could also spread from the surfaces into the metal reactorshroud wall (not shown).

FIG. 1 also shows oxygen and a noble metal penetrating into the IGSCC30. The metal reactor shroud 20 has an inner surface 10 with reactorcoolant flowing in direction 40 (shown left to right only as an exampleflow), together with both dissolved oxygen molecules 33 and noble metalmolecules 35. The oxygen molecules 33 and noble metal molecules 35 aredepicted as penetrating into the IGSCC 30. It is believed that the noblemetal molecule(s) 35 penetrate deeper into the IGSCC than do thedissolved oxygen molecule(s) 33. By doing so, it is believed that noblemetal molecule(s) (and/or particles thereof) prevent and/or otherwisemitigate the effects of the dissolved oxygen molecules 33 such aspotentially causing IGSCC 30 to form and/or propagate deeper or furtherinto the interior of the metal reactor shroud 20. The so describedeffect of the noble metal may be depicted by the “X” at 36 in FIG. 1

FIG. 2 is an example sample holder assembly containing three samples, inaccordance with the example embodiments of the present invention. InFIG. 2, a sample 200, which may be made of a metal or other suitablematerial, is shown. The sample 200 may be made of the same material usedto form the metal reactor shroud 20. The sample 200 shown in FIG. 2contains at least one formed IGSCC referred to herein as a thermalfatigue crack (TFC).

The TFC within the sample is intended to mimic an IGSCC within a metalreactor shroud. The sample 200 is intended to be placed in the metalreactor shroud 20 as described herein. In another example, the samplemay be placed at a distance of about 6″ or less from the interiorsurface of the metal reactor shroud wall, typically below the surface ofthe reactor coolant flowing in the metal reactor shroud 20 (e.g., atsome depth which may be variable so long as it is below the surface ofthe waterline within the shroud.

The sample 200 of FIG. 2 contains two TFCs 30 a and 30 b. However, 1, 2,3 or another multiple of TFCs may be provided in sample 200. Also, whilesample 200 is depicted as having a rectangular shape, the surface 10 aof sample 200 may be relatively flat and/or may mimic the curvature ofthe inner surface 10 of the metal reactor shroud 20. The sample 200could be of any shape, whether it is rectangular, or otherwise, so longas the shape permits the sample 200 to mimic the formation and/orpropagation characteristics of an IGSCC within a metal reactor shroudwall. Thus, any shape or configuration permitting the same may be used.

According to an example embodiment of the present invention, the sample200 may be made of a metal, for example, of carbon steel or stainlesssteel, some other steel-based alloy or some combination thereof.Furthermore, the metal of the sample 200 may be the same as the metalused to form the metal reactor shroud wall itself. It is possible thatthe metal reactor shroud wall is made of carbon steel and the metalsample 200 be made of stainless steel (e.g., 304L stainless steel or alow carbon steel) or vice versa.

In another example, the metal of the sample 200 may contain one or moreTFCs 30 a, 30 b having a variety of dimensions in length, width anddepth. For example, a thermal fatigue crack may have a length of atleast about 0.5 inches, a width of at least about 0.5 mils (0.0005″) anda depth of at least about 100 mils (0.1″). In one example, otherdimensions for the TFC 30 a, 30 b include a length of at least about0.75 inches, a width of at least about 0.001 inches and depth of atleast about 0.5 inches. Another dimension for the TFC 30 a, 30 b may bea length of least about 1″, a width of at least about 0.0015″, and adepth of at least about 0.75″. Examples of TFC dimensions provided asL×W×D are ½″×0.005″×0.1″; ¾″×0.001″×0.5″; 1″×0.0015″×0.75″; ½″×W×0.1″;¾″×W×0.5″; 1″×W×0.75″ where W=0.002″; 0.0025″; 0.003″; 0.0035″; 0.004″;0.0045″; or 0.005″. Other dimensions for the TFC within a sample may beused.

According to another example embodiment of the present invention, thesample 200 may be provided in a holder configured to hold one or moresamples therein. For example, the sample holder may be configured tohold 1, 2, 3, 4, 5 or more samples within. In an alternative, the sampleholder may be used to hold probes and/or other indicators or samplingdevices, for example.

According to another example embodiment of the invention, the metalsample 200 containing at least one TFC 30 a, 30 b may be placed in thesample holder which itself is then placed adjacent to an inner surfaceof the metal reactor shroud 20 at a place so that the metal sample 200itself is held below the surface of the reactor coolant. The reactorcoolant surrounds various fuel bundles within the nuclear reactor andcontacts the inner surface 10 of the metal reactor shroud 20 and surface10 a of the sample 200.

In nuclear reactors, such as boiling water reactors, at least fivestandard depth levels are recognized as depth levels H1, H2, H3, H4, orH5. Each of these levels represents a depth below the surface orwaterline of the reactor coolant, and spans beneath the waterline,anywhere between the core plate (bottom of fuel) and the bottom of thereactor core. The H1-H5 levels may represent successively deeper depthbands or regions below the reactor coolant waterline within the metalreactor

In one example, the H1 level may be a banded region anywhere from about0 to about 9 inches from the surface of the reactor coolant containedwithin the metal reactor shroud. The H2 level may be anywhere from about9 inches to about 40 inches from the surface (e.g., >9-40″). The H3level may be anywhere from about 40 inches to about 54 inches from thesurface (e.g., >40-54″), the H4 level may be anywhere from about 54inches to about 105 inches from the surface (e.g., >54-105″), and the H5level may be anywhere from about 105 inches to 194 inches from thesurface (e.g., >105-194″). The use of the sample holder is optional, solong as the sample 200 is otherwise maintained at a desired depth belowthe reactor coolant waterline within the metal reactor shroud 20.

According to one example, the sample 200 is submerged within the reactorcoolant and maintained therein for at least 2 weeks or more.Furthermore, one or more samples 200 (e.g., 2, 3, 4, 5, or more) havingone or more TFCs 30 a, 30 b (e.g., 2, 3, 4, 5, or more) may be placed inthe metal reactor shroud 20 as described above. If multiple samples 20are used, each may be removed at different time intervals to obtain astaggered temporal picture of the formation and/or propagation of IGSCCs30 in the metal reactor shroud 20 wall, as reflected by inspection ofthe one or more TFCs 30 a, 30 b in the one or more samples 200.

A purpose of maintaining the sample 200 in the metal reactor shroud 20within the reactor coolant for at least 2 is to expose the sample 200 tothe same conditions that are present within the metal reactor shrouditself. Therefore, as a sample 200 is removed from the metal reactorshroud 20 for examination, the sample 200 may provide a snapshot of thecondition of the metal reactor shroud 20 wall as it existed at the timethe sample 200 was removed for inspection.

A noble metal may be injected into the reactor coolant for a number orreasons, such as to stop formation and/or prevent or slow down thepropagation of an IGSCC within a metal reactor shroud 20 wall. A varietyof noble metals may be used for this purpose. For example, one examplenoble metal for use in conjunction with various example embodiments ofthe present invention include, but are not limited to, Pt, Rh, Pd, Ag,Au, Ir or a combination or combinations thereof. In one specificexample, Pt, Rh or combinations thereof may be used. In another example,a Pt/Rh mix may be used, which may vary in ratio from plant to plant,such as a 2:1 Pt to Rh ratio. In some plants, only Pt may be used as thenoble metal additive.

As previously described herein with regard to various exampleembodiment(s) of the present invention, it is believed that the noblemetal injected into the reactor coolant is carried by the reactorcoolant into an IGSCC 30 within the metal reactor shroud 20 wall andinto TFC 30 a, 30 b within the sample 200 that is held submerged withinthe reactor coolant. It is believed that the noble metal penetrates intothe IGSCC 30 and/or the TFC 30 a, 30 b to stop formation and/or reduce,minimize or slow down propagation of the IGSCC 30 and of the TFC 30 a,30 b within the metal reactor shroud 20 wall and the sample 200,respectively.

According to an example embodiment of the present invention, a noblemetal is introduced into the reactor fluid while the reactor is inoperation (e.g., generating power). The noble metal(s) may be injectedat a rate sufficient to maintain a level of at least about 100 parts pertrillion (ppt) in the reactor fluid (e.g., water) for a duration ofabout 2 weeks. Typically, this requires, for example, a noble metalinjection rate into the reactor fluid (e.g., feedwater) of about 0.3grams per hour. Other suitable injection rates, noble metal levels andmethods for introducing a noble metal into the reactor fluid may beused. Injecting of a noble metal may be carried out according to thedetails provided in commonly-assigned U.S. Pat. Nos. 5,600,961;5,608,766; 5,602,888; 5,818,893; 5,805,653; 5,130,080; 5,130,081;5,135,709 and 5,164,152, for example.

As shown in FIG. 1, the noble metal may counteract the deleterious IGSCC30 formation and/or propagation that are believed to occur due todissolved oxygen within the reactor coolant. This is shown by the “X”corresponding to element 36, for example.

According to another example embodiment of the present invention, thesample 200 submerged within the reactor coolant is removed at a giventime interval. The sample may then be analyzed for evaluating the statusof IGSCCs 30 formed in the metal reactor shroud 20. This may beaccomplished by examining the TFCs 30 a, 30 b within the sample 200.

FIG. 3 is a cut-out view of a portion of a metal reactor shroudincluding a sample holder; and FIG. 4 is a cut-away view showing onlythe sample holder without shroud. Referring to FIGS. 3 and 4, there isshown a holder 100 for holding one or more sample(s) 200. The holder 100may include a top guide clamp 50, a anchor clamp 60, a mechanism forholding sample(s) 200 within the holder 100, and a flow conditioner 70arranged below a location where sample(s) 200 may be held within thesample holder 100.

The flow conditioner 70 may have a length L and a depth D. The dimensionD may taper down to smaller values in the direction from top guide clamp50 to anchor clamp 60, as shown in FIG. 4, for example. The dimensionsof the flow conditioner 70 in terms of a ratio of L/D may provide for asmoother flow (as depicted by arrows 40) of the reactor coolant acrossthe surface 10 a of the sample 200. While the particular sample holder100 of FIGS. 3 and 4 is depicted, any other equivalent sample holder maybe used. A sample holder, however, is optional if the sample 200 can beheld in place where necessary or desired to adequately perform therequisite monitoring function.

Referring to the flow conditioner 70 in FIG. 4, the dimensions L and Dthereof may be adjusted so as to permit flow of reactor coolant past thesurface(s) 10 a of sample(s) 200 in relatively smooth, uninterruptedand/or undisturbed fashion. The flow conditioner 70 may provide adesired flow (e.g., in relatively uninterrupted, undisturbed and/orsmooth fashion) due to its wedge-shape, in which the depth D at the topof the flow conditioner 70 (closer to the top guide clamp 50) approacheszero (or is smaller) than at its bottom end (closer to the anchor clamp60). In one example, the ratio of L/D may vary from about 1:1 to about20:1. Also, other intervening L/D values may be used with flowconditioners 70 in conjunction with embodiments of the presentinvention. Thus, for example, other L/D ratios for the flow conditioner70 suitable for use in conjunction with example embodiments of thepresent invention may include a range of about 5:1 to 15:1, and aspecific ratio of about 12:1 for example.

Referring to FIG. 3, the sample 200 may be located adjacent to an innersurface 10 of the shroud wall 20, or at a distance of about 2-3″ or lessfrom the inner surface 10 of the metal reactor shroud 20 wall. In anexample, the distance between surface 10 and surface 10 a may be 6″ orless. In another example, the sample holder 100 may be positionedadjacent the inner surface 10 of the metal reactor shroud 20 wall co asto be within about 4″ of the nearest fuel bundle thereto (and/or withina range of about 4-10 inches from any fuel bundle) to hold the sample. Asecuring mechanism 80 may be provided for securing the sample holder 100with its sample(s) 200 in place, and a plug 95 may be provided forsecuring the anchor clamp 60 to the metal reactor shroud 20.

FIG. 5 a is an enlarged view of a portion of the sample holder showinghow a sample may be held within a cavity of the sample holder; and FIG.5 b shows a sub-holder portion of the sample holder in FIG. 5 a. FIG. 5a is a close-up view of a portion of the sample holder 100 with sample200 held in place, where the sample contains two TFCs 30 a, 30 b asshown. The sample holder 100 may include a rod 110 and a securingmechanism 120 to secure and maintain sample 200 in place. Other securingmechanisms may be used.

FIG. 5 b shows how sample 200 may fit into a cavity 100 b usingsub-holder 100 a. In FIG. 5 b, the cavity 100 b receives sample 200.Sample 200 is secured therein by searing mechanism 120, which isreceived through a base (not shown) on either side of the sub-holder 100a.

FIGS. 6 a-6 c illustrates the sample removed from the metal reactorshroud, where one or more cracks are split open so that the samplebreaks into two portions exposing an interior surfaces thereof.Referring to FIGS. 6 a-6 c, after having been exposed to the conditionswithin the metal reactor shroud 20 for a period of time during operationof a nuclear reactor, the sample 200 is removed and then the crack (30 aor 30 b) within the sample 200 is separated to split the sample intoparts 200 a and 200 b exposing interior crack surfaces (30′a and 30′b).

FIG. 7 illustrates an example process of using a scanning electronmicroscope (SEM) to reveal the deposition of a noble metal on surfacesof the crack. Thereafter, the interior surfaces (30′a and/or 30′b) ofthe TFC 30 a, 30 b (expected to mimic the characteristics of IGSCCs 30within the metal reactor shroud 20) are analyzed by a variety ofmethods, one of which may be analysis under a microscope showing a fieldof view 300 and revealing where noble metal molecules 35 (or particlesthereof) may have deposited within the TFC 30 a, 30 b on surfaces 30′aand/or 30′b. Instead of SEM inspection, stripping analysis may be used.

Acid stripping analysis refers to a dissolution process performed in achemistry laboratory to remove deposited noble metal from the surface ofa sample. For example, a sample loaded with noble metal is placed in abeaker containing a mixture of hydrochloric acid (HCl—e.g., at leastabout 15-25% by weight in water, such as 22%) and nitric acid(HNO₃—e.g., at least about 15-25% by weight in water, such as 20%) andbrought to a boil for a period of about 2 minutes. According to oneembodiment, the acid mixture used, for example, may be prepared bymixing a stock solution of HCl (e.g., 35-40% HCl by weight) with a stocksolution of HNO₃ (e.g., 80% HNO₃ by weight) wherein the mixture is a 3:1by volume mixture of HCl:HNO₃. Other suitable acid mixtures may be used.

The acid mixture dissolves the noble metal from the sample surface. Theacid beaker containing the sample may be then optionally placed in anultrasonic bath to further facilitate removal of the noble metal fromthe surface. The sample is then removed from the beaker and the acidsolution is diluted to, for example, 50 mL. That solution is thenanalyzed with inductively coupled plasma mass spectrometry (ICPMS) orother mass spectrometry with suitable resolution to determine the amountof the noble metal deposited in and/or around the TFC 30 a, 30 b. Othermethods suitable for determining or measuring the amount of noble metaldeposited in or around the TFC 30 a, 30 b may be used.

For example, SEM inspection may be conducted. SEM inspection refers toinspecting surfaces such as 30′a and/or 30′b under a scanning electronmicroscope at a suitable magnification and/or suitable wavelength toprovide or elucidate sufficient detail at surfaces 30′a and/or 30′bregarding the extent to which noble metal(s) may have deposited on suchsurfaces. If an insufficient amount of a noble metal is deposited onsurfaces such as 30′a and/or 30′b, the amount of the noble metal(s) thatmay be introduced or injected into the reactor fluid may be increased,decreased, or otherwise adjusted to achieve the desired level of noblemetal(s) to be deposited on surfaces 30′a and/or 30′b. By so adjustingthe amount of noble metal injected into the reactor fluid, thedeposition of noble metal into TFCs 30 a, 30 b and IGSCCs 30 is expectedto be improved.

According to an embodiment, noble metal is injected into the reactorfluid when it is believed that the amount of noble metal being depositedwithin an IGSCC is expected to be less than or equal to about 0.1μg/cm2. Typically, the maximum amount of noble metal that may beintroduced and/or injected into the reactor fluid should be no more thanabout 30 gm/year, which is the equivalent of no more than about 30μg/cm2 of noble metal depositing on the fuel rod cladding within thenuclear reactor.

The noble metal may also be introduced/injected into the reactor fluidwhen it is believed that the electrochemical corrosion potential (ECP)at the standard hydrogen electrode (SHE) is expected to be below about−230 mV. So, for example, if the ECP at the SHE is expected to be about−250 mV, then noble metal should be introduced or injected into thereactor fluid.

Pursuant to an embodiment, the noble metal may be injected at a rate ofabout 0.1 gm/hr, about 0.2 gm/hr, about 0.3 gm/hr, 0.4 gm/hr, and 0.5gm/hr or more as appropriate. If the set maximum (according to oneembodiment) of 30 gm/year is already met, then H2 may be added to thereactor fluid to adjust the ECP at the SHE to be above about −230 mV.

FIG. 8 illustrates a test stainless steel sample at an interior surfaceof an artificially created crack/crevice (formed using a stainless steelwasher as a shim) when the crack/crevice was exposed to water containing39 parts per billion (ppb) Pt. The boulder-like structures 900 representan oxide (e.g., iron oxide) formed on the interior surface of thecrack/crevice. The grainy particles 35 are deposited Pt particles

Having described various embodiments of the present invention, thefollowing examples are provided to illustrate various non-limitingaspects of the invention.

EXAMPLE 1

A TFC may be formed in a stainless steel (or other metal) sample by thefollowing procedure:

-   -   (1) Hold metal sample and pull two ends of sample with a load        sufficient to apply a tensile stress to the sample;    -   (2) Apply hot and cold material to sample under tensile stress        in alternating (or other suitable) manner sufficient to initiate        and/or propagate a TFC in the sample. This application of hot        and cold material may include the use of a jet of extremely hot        water followed by a jet of extremely cold water.    -   (3) Repeat step(s) (1) and/or (2) as needed to yield a TFC        having the desired dimensions in terms of length, width and        depth thereof.

Having described various embodiments, the following claims are appendedbelow. The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A method for monitoring deposition of a noble metal in anintergranular stress corrosion crack (IGSCC) in a metal reactor shroudwall of a nuclear reactor comprising the steps of: placing a metalsample at a location within about six inches or less of an inner surfaceof the metal reactor shroud wall, the sample submerged below a waterline in the reactor, the metal sample containing at least one thermalfatigue crack, maintaining the sample at the location for a givenduration, introducing a given amount of the noble metal into reactorwater while the sample is maintained at the location, and removing thesample from the location.
 2. The method of claim 1, wherein the sampleis maintained at the location for at least two weeks.
 3. The method ofclaim 1, wherein the thermal fatigue crack has an opening of at leastabout 0.0005 inches.
 4. The method of claim 1, wherein the thermalfatigue crack has an opening in a range of about 0.0005 to 0.005 inches.5. The method of claim 1, further comprising: analyzing deposition ofthe noble metal in the thermal fatigue crack.
 6. The method of claim 5,wherein the analyzing is conducted by acid stripping analysis.
 7. Themethod of claim 5, wherein the analyzing is conducted by SEM inspection.8. The method of claim 6, wherein introducing the noble metal includesadjusting the amount of the noble metal being introduced based onresults of the acid stripping analysis.
 9. The method of claim 7,wherein introducing the noble metal includes adjusting the amount of thenoble metal being introduced based on results of the SEM inspection. 10.The method of claim 1, wherein said noble metal is Pt, Rh, Pd, Ag, Au,Ir or a combination thereof.
 11. The method of claim 1, wherein thenoble metal is one of Pt, Rh or a combination thereof.
 12. The method ofclaim 1, wherein introducing further includes injecting the noble metalinto said water.